Demodulators – Amplitude modulation demodulator – Input signal combined with local oscillator or carrier...
Reexamination Certificate
1998-07-07
2001-01-30
Kinkead, Arnold (Department: 2817)
Demodulators
Amplitude modulation demodulator
Input signal combined with local oscillator or carrier...
C329S361000, C342S051000, C455S084000, C455S337000, C340S870030
Reexamination Certificate
active
06181198
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an amplitude and phase demodulator circuit for signals with very low modulation index. The invention is related to the first stages of receiver circuit RX which have the purpose of optimally demodulating and operating a first amplification of a demodulated signal. More particularly, the invention is suitable to be applied to communication systems between a “transponder”
2
and a base station
1
, as schematically depicted in FIG.
1
.
2. Discussion of the Related Art
The base station includes a transmitter TX that generates a carrier whose frequency is generally 125 KHz. This in turn generates, by means of the tuned circuit C
1
-L
1
, a magnetic field in the coil L
1
working as an antenna.
A second inductor L
2
is placed in this magnetic field, working as an antenna, connected to a second tuning capacitor C
2
and to an electronic circuit. This one includes a secret code and a circuit which is able to modulate the voltage of the resonant circuit L
2
-C
2
with a sequence of high and low values, which correspond to the sequence of binary digits (bit) composing the secret code itself.
If the inductor L
2
is placed to a distance sufficiently close to L
1
, but without the need of electromechanical contacts between the two, a magnetic coupling M between L
1
and L
2
appears, which is sufficient to generate at the ends of L
2
-C
2
a voltage for supplying the transponder electronic circuit
3
. This supply system which needs neither batteries nor contacts may be called “remote supply”.
The transponder internal electronic circuit is supplied by the AC voltage at the terminals of L
2
-C
2
, which is by itself properly rectified and smoothed, and is able to transmit the code included in its memory. To do this the electronic circuit absorbs either a high or a low current from the resonant circuit L
2
-C
2
, in accordance with the binary value, respectively low or high, to be transmitted.
This current consumption modulation is applied to L
2
-C
2
and propagates to the resonant circuit L
1
-C
1
, attenuated by the low coupling coefficient associated with the mutual inductance M.
The Vrx signal on the resonant circuit L
1
-C
1
is provided by the TX transmitted carrier, hence with a rather high level, and by a modulating component as a result of that explained above.
The Vrx signal is sent to the input of an RX reception circuit which has the purpose of carrying out a demodulation and hence of reconstructing the data of the transponder memory. This data represents the secret code which is subsequently interpreted by a microcomputer &mgr;C.
The RX circuit is generally able to process signals with voltages not any higher than 5V, for reasons of economy of the materials with which it is built. Hence its input signal Vrx must be first attenuated if it is not included within such limits. Though, this means that the modulating signal will typically be 5V/1000=5 mV, but actually RX will have to guarantee good performance with modulating signals as low as 1 mV.
The Vrx signal is typically amplitude modulated, but because of misalignments among the resonance frequencies of the tuned circuits L
1
-C
1
and L
2
-C
2
and the excitation frequency coming out from TX, phase modulation components may appear as well.
FIG. 2
shows the amplitude and phase Bode plots of the resonator L
1
-C versus the excitation frequency that in our case happens to be the carrier output from TX.
The plots show two curves both for amplitude and phase, which correspond to the two cases of current consumption at the transponder side.
If the resonator is well tuned with respect to the carrier frequency (case
1
) the two levels cause amplitude variation but no phase variation. But, if there is a small misalignment between resonant and excitation frequencies (case
2
), a phase modulation appears together with the amplitude modulation. Finally, if the misalignment becomes wider (case
3
), then the amplitude modulation disappears and the phase modulation remains.
FIGS. 3
,
4
, and
5
show the waveforms versus time for cases
1
,
2
, and
3
respectively.
The sensitivity of a receiver substantially depends on its equivalent input noise, to which the first amplifier stage of the receiver chain contributes, together with all those stages between this one and the receiver input which do not provide meaningful amplification, as for example a demodulator can be.
FIGS. 6A
,
6
B and
6
C show an example of prior art in which the input signal Vrx immediately becomes amplified, then demodulated. However, the demodulation circuit must work at high voltages, therefore it has the inconvenience of having low performance as it must be simple, or else it would be too expensive. This solution can only demodulate the amplitude but not the phase, hence case
3
of FIG.
2
and
FIG. 5
cannot be handled.
FIGS. 7A
,
7
B and
7
C show a second example of prior art in which the Vrx signal becomes first demodulated with a multiplication with a square wave (at the mixer node
4
) which is synchronous (SYNC) with the transmitted carrier (obtaining a Va signal), then it becomes smoothed by a low-pass filter
5
that eliminates the residual carrier frequency component (125 KHz typical), but that doesn't affect the base band signal (100 Hz-5 KHz typical).
The resulting Vb signal may have a DC component as high as the amplitude of the Vrx input signal, hence no amplification is possible before this point. A subsequent high pass filter
6
, with a cut off frequency that is lower than the base band lower limit (100 Hz typical), may eliminate the high DC component and simultaneously amplify and obtain a useful signal Vout.
The SYNC signal must have a proper phase with respect to the carrier as shown in FIG.
8
: accordingly, a maximum difference between the two levels, high and low, of the demodulated signal is reached. Often the search for the optimum demodulation phase is carried out by an algorithm that is implemented by a microprocessor.
This kind of solution is widely used, though it has the drawback of having a certain number of elements that, located before the high pass filter with first amplifying stage, contribute to the equivalent input noise by limiting the input sensitivity.
The main sources of noise are:
the phase jitter of the SYNC signal, with the maximum effect right in the case of perfect alignment of the two antennas with the carrier frequency, where the phase component modulation is absent;
the mixer, which is a simple voltage follower with a switching gain between +1 and −1 according to the commanding SYNC signal;
the low pass filter.
FIGS. 9A and 9B
show a third example of a prior art circuit in which the input signal Vrx is demodulated by means of sampling with a signal (SYNC) which is synchronous with the transmitted carrier and suitably phased with respect to such carrier.
The sampled signal Va is then stored by a block
7
which uses the same SYNC signal and which has a low-pass filtering feature: accordingly, the signal is smoothed and the derived signal, Vb, is then pass-band filtered in a filter
8
(with a gain Av) to cancel the DC component of the signal to obtain the Vout signal.
The advantage of this system is represented by the fact that the sampling takes into account the only portion of the Vrx signal in which the difference between the two logic levels (high and low) is maximum, i.e., the information content is higher.
Hence a better signal to noise ratio is achieved. Moreover, the blocks before the first gain stage Av, that carry out the sample and hold action, may be simply realized for example with capacitors and switches, thus contributing to a low equivalent input noise.
A further advantage is that the phase jitter of the synchronism signal (SYNC) is not converted to noise when Vrx only shows the amplitude modulation component (thanks to a good frequency alignment of the carrier with the two antennas) and the sampling instant occurs at the maximum or minimum peak of the Vrx sinusoid.
It must be n
Poletto Vanni
Sass Dieter
Galanthay Theodore E.
Kinkead Arnold
Morris James H.
STMicroelectronics S.r.l.
Wolf Greenfield & Sacks P.C.
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